A mine dump truck or the like is provided with a retarder (a continuous braking device), which is controlled to keep a vehicle speed constant while the truck or the like is descending a long downhill road. There are various types of retarders: an electromagnetic type, an exhaust type, an air type, a hydraulic type, and so forth, and an electromagnetic type, a hydraulic type, and the like are cooled type.
For example, in an oil-cooled multiple disc type retarder, a plurality of vehicle body side plates (fixed side plates) and a plurality of wheel side plates (rotating side plates) are alternately placed adjacently to each other to be separable from each other, and cooling oil flows between both plate surfaces. While braking with the retarder (specifically, when the retarder is applied), a negative pressure (an oil pressure Pp) is given to the end plate to allow all the plates to slide while the surfaces of all plates contact one another, whereby obtaining a sliding friction force as a braking force. In other words, the more the oil pressure Pp is increased, the more the braking force increases, and the more the circumferential speed of the wheel side plates decreases. The oil pressure Pp is adjusted by means of a retarder lever in a manual control type, while in an automatic control type, it is adjusted by a command from a controller. In this situation, braking energy is proportional to the product of the oil pressure Pp and the circumferential speed of the rotating side plates, and comes out as heat generation. This heat generation is absorbed in cooling oil, and the cooling oil is cooled by means of a heat exchanger. The cooling oil from the heat exchanger flows between the plates again by means of a hydraulic pump rotated by an engine. Consequently, in a cooled type retarder, a vehicle speed is controlled to remain constant and not more than a maximum vehicle speed Vmax at which the heat balance of the vehicle and the retarder can be maintained. The explanation is made below with the above oil-cooled multiple disc type retarder taken as an example.
The control of the vehicle speed to remain constant by means of the retarder is as follows in concrete. A manual control is taken as an example in the description. In view of the loading weight, the gradient and the distance of a downhill road, an operator previously knows the maximum vehicle speed Vmax at which the heat balance can be maintained. When applying the retarder while descending a downhill road, the operator initially coasts the vehicle and manipulates a retarder lever so that the vehicle speed remains not more than the maximum vehicle speed Vmax and constant in the coasting state. The manipulated variable of the retarder lever is proportional to the braking force of the retarder, and the braking force is inversely proportional to the vehicle speed. In this situation, the operator shifts a transmission lever to a gear of the transmission with which the desired vehicle speed V (.ltoreq.Vmax) is obtained. In a vehicle having a plurality of gears and an automatic transmission which automatically shifts the gear based on the vehicle speed V, the automatic transmission automatically shifts to a gear with which the vehicle speed V is obtained. It should be noted that the above "coast" means the state in which the operator removes his or her foot from the accelerator pedal to allow the vehicle to travel by inertia.
In the above cooled retarder, the following are known as an automatic retarder controller having a switch system switch for switching between the manual control and automatic control and various detectors, receiving a switching signal to the automatic control from the aforementioned switch system, and thereafter automatically controlling, the retarder based on detected information from the detectors.
An automatic retarder controller in the first example has an engine speed detector, and automatically controls the retarder so that the engine speed from the engine speed detector matches a reference engine speed previously stored, irrespective of the manipulated variable of the retarder lever.
Only one reference engine speed is set in the above first example. However, firstly, there is a disadvantage that the vehicle speed is greatly fluctuated according to the slippery conditions of a road surface, the loading weight, the gradient of a downhill road, and the like even at the same engine speed. Secondly, since the same engine speed exists in each gear, if abrupt braking is exerted by means of a foot brake, for example, the speed decreases to shift down to a lower gear, and matches the reference engine speed of the gear, thereby causing substantial reduction of speed. Likewise, if an accelerator pedal is depressed, the speed increases to shift up, and matches the reference engine speed of the gear, thereby causing substantial increase in the speed. Specifically, there is the disadvantage of causing speed change shock. Thirdly, the reference engine speed in the highest speed gear in the position of the shift lever is controlled to remain constant, which makes it impossible to finely change the setting of the engine speed during, traveling. Specifically, when the accelerator is depressed to cause the engine speed to exceed a specified engine speed, the gear shifts to a higher gear, and thereby the engine speed matches the reference engine speed in the gear, which causes a substantial increase in speed. Fourthly, the manipulation of the retarder lever shifts the gear to a lower gear, but the control is not performed until the engine speed reaches the reference engine speed in the maximum speed gear in the position of the shift lever, and therefore the brake control is not performed even the gear is shifted to a lower gear. Incidentally, if a brake, a foot brake pedal, a parking brake are operated other than the retarder lever, the result is the same.
The automatic retarder controller in the second example has a vehicle speed detector, and automatically controls the retarder so that the vehicle speed from the vehicle speed detector matches the reference vehicle speed previously stored, irrespective of the manipulated variable of the retarder lever.
As for the above controller in the second example, the operator previously inputs a reference speed corresponding to the optimum maximum speed Vmax in accordance with a loading weight, the gradient and the distance of a downhill road, and the like, and the automatic retarder controller controls the vehicle speed to remain constant based on the above vehicle speed. Accordingly, if a plurality of downhill roads exist in a course, it is necessary to input reference vehicle speeds respectively for the downhill roads one by one, thus causing the disadvantage of requiring much time and effort.
The automatic retarder controller in the third example has a retarder oil temperature detector, and decreases an oil pressure to prevent continuous overheating when the retarder oil temperature obtained from the retarder oil temperature detector reaches the overheating temperature previously stored, irrespective of the manipulated variable of the retarder lever.
The above third example has the disadvantage of reducing the braking force due to the decreased oil pressure, which increases the vehicle speed.